MEMS (Micro-Electro-Mechanical systems) mirrors have wide applications in fiber optic networks, such as optical cross connect switches, optical attenuators, optical tunable filter etc. The most mature MEMS product in optical telecommunication industry is MEMS Variable Optical attenuator (VOA).
There are a number of MEMS VOAs disclosed in US patents. A micro shutter type MEMS VOA is disclosed in U.S. Pat. Nos. 6,275,320B1, 6,459,845B1, 6,751,395B1, 6,780,185B2, 6,816,295B2, 6,876,810B2, 6,901,204B2, 6,954,579B2, 6,980,727B1, 6,996,306B2 and 7,224,097B2. These VOAs use a micro shutter to partially block a light beam in order to achieve optical attenuation. These micro shutters are actuated by such means as electro-thermal actuation or electrostatic actuation. Micro shutter type of MEMS VOAs has difficulties, such as optical component alignment and hermetical packaging.
Micromirror type MEMS VOAs have advantages of simple packaging. The optical attenuation is realized by the tilting micro mirror, which redirects the light beam. The commercially available lens and TO metal cans can be readily available for low cost packaging of micro mirror type VOAs. As such, most of the commercial available MEMS VOAs use a tilting micro mirror. U.S. Pat. Nos. 6,628,856B1, 6,838,738B1, 6,915,061 6,963,679 and 7,224,097B2 disclose MEMS micro mirrors. These micro mirrors use electrostatic actuation. The electrostatic actuation is favored for micro mirror due to its low power consumption and relative small footprint.
In the disclosed prior arts, micro mirrors with electrostatic actuation fall into vertical combdrive type and parallel plate type. U.S. Pat. No. 6,838,738B1 disclosed vertical combdrive actuated micro mirror, it has several drawbacks of device design and fabrication. First of all, the design of the taller and shorter fingers using the same layer of material have some initial overlapping areas, which will have effect to against the actuation. The electrical field in this initial overlapping area has opposite contribution to the mirror actuation. Secondly, the micro mirror is required to have a certain minimum thickness to maintain its mechanical strength to overcome the residual stress of the reflective metal film on its top surface as well as environment vibration during its operation etc. Thinner than 20 microns of material will cause undesirable higher radius of curvature (ROC) of the micro mirror. If 20 microns of thick material is used to make taller and shorter fingers, it is very difficult to have good photolithography in its process step 840 since higher topography created in the previous step (step 830). Even if the photolithography can be managed, then the finer finger gap has to be sacrificed, which in turn results in higher actuation voltage. Thirdly, one metal coating is used in U.S. Pat. No. 6,838,738B1 for both reflective metal film on the mirror surface and metal film on the bonding pads for wire bonding. The requirements for both metal films are quite different. The requirements for the reflective metal film on the mirror surface are higher reflectivity within the light wavelength interested and low residual stress. Usually this metal film is very thin for easy residual stress control. On the other hand, the requirements for the bonding metal film on the bonding pads are thicker metal film for easy wire bonding and good electrical conductivity. Usually this bonding metal film is thicker and stressful. One metal coating process in U.S. Pat. No. 6,838,738B1 will cause either higher ROC of the micro mirror, poor reflectivity and/or poor wiring bonding. Fourthly, U.S. Pat. No. 6,838,738B1 disclosed the wet structure release processing step 890, which will cause stiction of the macrostructure such as fingers. Stiction will lead to defective devices. Last but not least, due to the existing and unavoidable process defects, the vertical combdrive actuator has tendency to rotate side ways so that the electrical shorting will occur from contact of fixed and movable fingers. Such electrical shorting can permanently destroy the device. There is no indication in U.S. Pat. No. 6,838,738B1 as to how to prevent undesirable side way rotation.
Compared with vertical combdrive actuator, parallel plate electrostatic actuators have following several disadvantages in all the prior arts. First of all, the pull-in effect of parallel plate electrostatic actuator of micro mirror limits the controllable tilting angle range under the certain actuation voltage. When actuation voltage is applied between fixed electrode and the movable hinged mirror, the resulting attractive electrostatic force will pull the mirror towards the fixed electrode to create tilting of hinged mirror. Initially, the mechanical restoring force from deformed hinge will balance the electrostatic force to keep the mirror in the controllable position. But when the actuation voltage is further increasing, and the tilting of the hinged mirror is over one third of the initial gap between the fixed electrode and the mirror, the electrostatic force between the electrode and the mirror surpasses the mechanical restoring force of the hinge, the hinged mirror will snap and physically contact to the fixed electrode. The usable and controllable tilting range of the mirror is very limited, only one third of the gap between the mirror and fixed electrode. Secondly, within the small controllable titling range, parallel plate electrostatic actuator won't provide linear actuation. In other word, the mirror tilting angle is not linear with the actuation voltage. Thirdly, higher actuation voltage causes issues of electrical charging, tilting angle drifting. In order to have larger controllable titling angle of the mirror, the gap between the fixed electrode and mirror has to be increased. Increased gap results in the higher actuation voltage. Higher driving voltage causes electrical charging on the dielectrical materials of the micro mirror device, which will in turn cause the undesired tilting angle drifting of the mirror. Fourthly, squeezed air between movable mirror and fixed electrode during tilting will lead into air damping. Since the space between the movable mirror and fixed electrode is very small, the fast titling/switching of the micro mirror will cause the air between its mirror and electrode either compressed or decompressed. As such, the air damping from the squeezed air will effectively lower the tilting/switching speed of the mirror. Lastly, the micro fabrication process is costly and complex, especially for making complex actuation electrodes and electrical wirings of the micro mirrors.